1. Field of the Invention
The present invention relates to a process for obtaining essentially carbon or carbonate-free inorganic nitrates or oxides. Specifically, the present invention relates to the treatment of a carbon-containing organic or inorganic salt or a mixture of salts with a nitrate-generating group, e.g. NO.sub.2, at essentially anhydrous
conditions between about 40 and 200.degree. C. to produce a solid carbon or carbonate-free nitrate or mixture of nitrates. The process is particularly useful to produce a mixture of oxides, e.g., YBa.sub.2 Cu.sub.3 O.sub.7-d, (where d is between about 0 and 0.7) having high temperature (at or above 90K) electrical superconducting properties, or high performance materials, such as BaTiO.sub.3 or barium aluminosilicate (celsian).
2. Description of the Related Art
Various superconducting materials have been produced by processes ranging from mixing and firing oxides, nitrates and carbonates (shake and bake) (M. K. Wu et al., Physical Rev. Letters, Vol. 58 pg. 908 (1987)), chemical-vapor deposition (P. H. Dickinson et al., Applied Physics, Vol. 66, #1, p. 1 (July 1989)); sol-gel (P. Barboux et al., MRS Fall Meeting Abstracts, 1987; coprecipitation (A. M. Kini et al., Inorganic Chemistry, Vol. 26, Pg. 1834 (1987)); aerosol pyrolysis techniques (T. T. Kodas et al., Applied Physical Letters, Vol. 52 #19 pg. 1622 (1988)); or freeze drying techniques (Johnson et al., U.S. patent application Ser. No. 071,977, filed Jul. 10, 1987, now U.S. Pat. No. 4,975,415, issued Dec. 4, 1991).
K. J. Leary et al., in the Journal of the American Ceramic Society, Vol. 73 (#4), pg. 904-908 (April, 1990) disclose the use of nitrogen dioxide to improve the superconducting properties of YBa.sub.2 Cu.sub.3 O.sub.7-d (where d is between about 0 and 0.7) (also referred to herein as YBaCuO or as YBCO). However, the nitrogen dioxide is only contacted with the oxide at temperatures of 600.degree. C. or greater for long times. Also see Leahy
R. B. Cass in U.S. Pat. No. 4,931,213 discloses the preparation of electrically-conductive titanium suboxides.
All references, patents, articles, standard, etc. cited herein are incorporated by reference in their entirety.
The production process may be fairly straight forward such as the shake and bake, or freeze drying approach or be somewhat more complex such as the sol-gel or coprecipitation. A problem encountered in the formation of barium containing superconducting oxides is that barium inorganic salts often have low water solubility. This makes it difficult to obtain proper solution concentrations for the freeze-drying technique described above. To circumvent this solubility problem, organic barium salts (acetates) are fairly water soluble and are used. However, these methods may be suspect because of the observations that the superconducting inorganic oxide may have undesired residual amounts of carbon present even after calcining, annealing and slow cooling. The undesired carbon present in inorganic oxide powders is obtained from carbonates any or other organic carbon moieties. The carbon present in any form is believed to diminish superconducting properties. Therefore, a major problem in most bulk processes to produce a superconducting oxide, or an advanced performance material, is the necessary removal of all potentially contaminating carbon moieties.
The present process provides a nitrogen dioxide, pretreatment method to remove carbon-containing organic and carbon-containing inorganic moieties from a carbon-containing precursor, nitrate or oxide prior to sintering, calcining, slow cooling, annealing, and slow cooling to produce the carbon free oxide. These carbon-free nitrates and oxides are particularly useful to produce improved superconducting oxides or advanced materials.